Receivers based on the direct conversion architecture are very appealing because they require fewer components and therefore are less complex than traditional super-heterodyne receivers. Direct conversion receivers however suffer from a very serious problem, which is DC-offset. Direct conversion receivers take a radio frequency (RF) signal and translates it into base-band. So if there is any DC-offset added to the received signal by the mixer (or any other component in the receiver), the DC component of the received signal will be lost because there is no intermediate frequency (IF) filter. Direct conversion receivers are very susceptible to the presence of inteferers (blocking signals within the receive band) that through self-mixing can produce a DC-offset that is added to the received signal.
If the received signal has zero mean, and the DC-offset remains constant, it is possible to solve this problem by averaging the received signal along a sufficiently long period, and then removing that average from the received signal. The DC-offset problem worsens when the DC-offset varies with time. If the DC-offset variation is slow, it is possible to update the estimated DC-offset by repeating the averaging process after a certain period or in a continuous manner. Temperature changes, etc can cause slow variations of the DC-offset. However, if the DC-offset varies too fast, the former procedure will stop working, because a signal can have zero mean over its total duration, but locally it will not have zero mean unless the signal is constant and zero. When the DC-offset changes very fast, it is called a time-varying or dynamic DC-offset, as opposed to the static or slowly-varying DC-offset which can be solved using a technique as described in U.S. Pat. No. 5,422,889 and entitled Offset Correction Circuit.
Because a blocking signal can appear at any time, the DC-offset component that will be added to the received signal can also appear at any time. As a consequence it is called “dynamic” DC-offset, because it will change depending on the arrival time of the blocking signal. This phenomenon is usually referred to as AM suppression. A typical dynamic DC-offset profile is shown in FIG. 2 for the I (in-phase) component of the signal. Signal 202 shows a GMSK I signal which is the desired signal. Signal 204 is the distorted signal having been affected by a DC offset. Line 206 highlights the dynamic DC-offset. The AM delay is shown by line 208, while the AM level is shown by line 210.
In the presence of a time varying DC-offset, the receiver performance degrades quite a bit with the amount of degradation depending on the DC-offset variation and the speed of the variation. That is why solving for time-varying DC-offset problems is one of the more critical problems to be resolved in a direct conversion receiver. One of the main reasons for the apparition of time-varying DC-offsets in TDMA direct conversion (also called homodyne) receivers is the presence of TDMA interferes, that through self-mixing or even order non-linearities, can produce a DC-offset that is added to the received signal. This is something that does not happen with super-heterodyne receivers. Because such an interferer can appear at any time, the DC-offset component that it will add to the received signal can also appear at any time. As a consequence, it is called a “time-varying” DC-offset, because it will change depending on the arrival time of the interfering signal.
Currently one of the most successful wireless communication systems in the world is the GSM system. GSM devices, such as portable radiotelephones (also called handheld devices), must pass tests to prove conformance with the GSM standard. One of those tests is called the “AM suppression test”. To pass the AM suppression test, the bit error rate (BER) performance of the handheld must not be degraded significantly in the presence of a very powerful TDMA interferer signal. It is clear from the previous explanation that a homodyne receiver will not be able to pass the AM suppression test if the receiver does not implement a good time-varying DC-offset correction method, because the interferer signal will generate a time-varying DC-offset that will be added to the received signal. A need thus exist in the art, for a method for correcting for a time varying DC-offset.